Generally, composite materials are prepared by imbedding a reinforcing material within a matrix material. Composite materials having high degrees of utility typically exhibit mechanical or other properties superior to those of the individual materials from which the composite was formed. A common example of a composite material is fiberglass. Fiberglass is glass fibers, which are the reinforcing material, embedded in a cured resin, which constitutes the matrix material.
Composite materials have been found to have a high degree of utility when used as parts of structures, components, sub assemblies, and the like, of assemblages such as aircraft, missiles, boats, medical equipment, and sporting goods. A composite commonly used in such applications is fiberglass. Other composites having particularly high degrees of utility in such applications are those that are prepared from carbon fibers combined with a matrix material such as thermoset (e.g. thermosetting and the like) and/or thermoplastic resins. Such composites are referred to as carbon fiber composites (herein after referred to as CFC), or more commonly, carbon composites. Carbon composites have been used, for example, as aircraft flight surfaces, missile bodies, orthopedic supports, and golf club shafts. The utility of such carbon composites is typically related to their exceptionally high strength to weight ratio and their fatigue and corrosion resistance. In most instances, these beneficial properties exceed those of the metals or other materials supplanted by the use of the carbon composites. Additionally, some types of carbon fiber composites can be carbonized to form carbon-carbon composites.
Specific fiber orientations may be desired in the final composite product to impart accentuated strength, stiffness, and/or flexibility along certain axes. Furthermore, composite forming materials, particularly carbon fiber, are relatively expensive and wastage is generally discouraged. For these and other reasons, composites are produced in sizes, shapes, and forms that closely match those required by the intended application. In fact, composites, particularly carbon fiber composites used in aerospace and many other applications, are routinely produced, within very restrictive tolerances, to the required size.
The forming of composites, including carbon composites, to such high dimensional requirements is typically accomplished by the use of mold like devices commonly referred to as tools. These tools encompass one or more surfaces, referred to as tool faces, upon which the composite is formed, shaped, molded, or otherwise produced into components of predetermined sizes and shapes. Such components can include structures, parts, sub assemblies and the like, and may be referred to collectively as parts. The tool face is a surface typically formed such that it is a precise three dimensional negative mirror image of a surface of the desired composite component. That is, a raised surface on the composite part will be matched and formed by an equivalently (negatively) dimensioned surface depression of the tool face. Likewise, a recessed surface on the composite part will be matched and formed by an equivalently (negatively) dimensioned raised surface of the tool face. In practice, a mixture of a reinforcing material and a matrix material, for example carbon fiber and a resin, are placed upon the tool face by any number of procedures and brought into intimate contact with that tool face. The dimensions of the tool face are such that this contact effectively molds a surface of the matrix material and reinforcing material mixture into the desired shape and dimensions. The matrix material is then solidified, typically by curing of the resin, to produce the composite component. For example, a carbon fiber containing resin is cured, typically by the application of heat, to yield a solid CFC component having a surface exhibiting the shape and dimensions imparted by the tool face.
In addition to the tool face, a tool is also comprised of a tool body and sometimes a support structure. The tool body comprises the tool face. That is, the tool face upon which the composite, for example a CFC, is formed is a surface of the tool body. The tool body may also encompass a cover which minimally encloses the tool face, or a portion thereof, such that an essentially closed volume is formed between the tool face and the cover. The support structure may be connected to the tool body and may serve a number of purposes, including but not limited to, support, orientation, and transportation of the tool body and face along with protection of the tool body and face from damage.
Important characteristics of tooling include, for example, quality, dimensional accuracy, weight, strength, size, cost, ease of repair, and the like. Additionally, rigidity and durability are considered to be very important characteristics of tooling. All of these characteristics are dependent on the tool design, the materials of construction of the tool, and on the materials used to form the composite.
A characteristic of the tooling that may be very important is the coefficient of thermal expansion (herein after referred to as CTE and CTEs in the plural form) exhibited by the tool face. As the tool face is a surface of the tool body, the CTE exhibited by the tool face is dependent on the material of which the tool body is comprised. It is generally desired that the tool face exhibit a CTE that is substantially similar or equivalent to the CTE of the formed composite component (which may also be referred to as a composite part, or more simply part) formed thereon. Preferably, the CTE exhibited by the tool face should be substantially similar or equivalent to the CTE of the formed composite part over a wide temperature range. The importance of having a substantial similarity, or more preferably equivalence, between the CTE of the composite part and that exhibited by the tool face is related to the manner in which composite parts are prepared using tools. That is, typically, the materials used to form the composite are placed on the tool face at room temperature. The temperature of the tool and composite forming materials is then increased to some elevated temperature, typically such as 250° F. or more, to cure the resin of the composite material. Once the resin is cured, the resulting composite part, for example a CFC, is rigid. Following resin curing, the tool face and composite part are cooled to room temperatures. Such exposure to temperatures significantly above room temperature is the reason it is desired that the CTE of the tooling match that of the resulting composite part. For example, if the CTE of the composite part is significantly less than that exhibited by the tool face, the composite part may be trapped or retained on the tool by the relatively greater contraction of the tool face dimensions with cooling. Conversely, if the CTE of the composite part is significantly greater than that exhibited by the tool face, the part may again be retained on the tool or may damage the tool face during contraction, or cured composite dimensions may differ from those of the tool face.
Typically, carbon composites have relatively low CTEs while the CTEs for most other materials are much higher. Therefore it is very difficult to match the CTE exhibited by the tool face with the CTE of a carbon composite as there are few materials available for construction of the tool body that have sufficiently low CTEs. Such available low CTE materials suitable for construction of the tool body include, for example, graphite, other carbon composites, INVAR® (e.g., a controlled expansion nickel iron alloy), and the like.
INVAR® is durable and has a CTE that is substantially similar to that of carbon composites. However, INVAR® based tools are typically heavy, difficult to fabricate, and can require, for example, as many as seventeen separate stages to fabricate. Such numerous fabrication stages can lead to about a 140% to about a 250% increase in tooling costs and a four fold increase in lead times, as discussed in “Fabrication and Analysis of Invar Faced Composites for Tooling Applications”, Proceedings of Tooling Composites 93, Pasadena, Calif., which is hereby incorporated by reference in its entirety.
As with INVAR® based tooling, graphite based tooling may be capable of matching the CTE of CFC parts, and the like, even for example, the difficult to match CTE of low CTE materials. However, such tooling typically utilizes relatively large blocks of graphite. Graphite is a relatively dense material. Therefore, such tooling may be relatively massive and possibly expensive. Additionally, such large, dense, massive, blocks of graphite may exhibit relatively high heat capacities and/or require extended time periods to heat and cool uniformly.
Similar to INVAR® based tooling, carbon fiber composite based tooling may be capable of matching the CTE of CFC parts, and the like, even for example, the difficult to match CTE of low CTE materials. For this type of tooling, carbon fiber composites are used as the total tool body and/or that portion of the tool body defining the tool face. Carbon fiber composite based tooling is advantageous as such CFC based tools are less expensive, lighter, have low thermal mass, and require shorter lead times for tool manufacturing than does conventional tooling such as that based upon INVAR®. However, CFC based tools are usually susceptible to damage if not handled with care, especially when composite is laid thereon. Additionally, surface degradation of CFC based tools may occur as a result of the repetition of the process cycle due to a combination of component adhesion, CTE mismatch, and oxidative decomposition. Furthermore, any necessary repairs of CFC based tools leads to an increase in repair and maintenance costs. Also, CFC based tools are subject to dimensional stresses from uneven support. Such stresses may cause a loss of rigidity. Accordingly, due to the aforementioned problems, CFC based tooling is not commonly used.
There are other important characteristics of composite tooling, particularly tooling for the production of CFC, which should also be considered. For example, in addition to being rigid, durable, strong, and CTE matchable, the tooling should also be low cost and easy to produce. That is, a factor usually considered when selecting material for a tool body is the total number of parts to be produced. Included in this consideration is the fact that production of large numbers of parts can more easily justify expensive tooling. Overall, however, it is generally accepted that rigid, strong, durable, and CTE matchable tooling, which can be easily produced at low cost, irrespective of the planned number of parts, is desired.